The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Internal combustion engines, especially automotive internal combustion engines, generally fall into one of two categories—spark ignition engines and compression-ignition engines. Traditional spark ignition engines, such as gasoline engines, are known to function by introducing a fuel/air mixture into the combustion cylinders, which is then compressed in the compression stroke and ignited by a spark plug. Traditional compression-ignition engines, such as diesel engines, typically function by introducing or injecting pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke, which ignites upon injection. Combustion for both traditional gasoline engines and diesel engines involves premixed or diffusion flames that are controlled by fluid mechanics. Each type of engine has advantages and disadvantages. In general, gasoline engines produce fewer emissions but are less efficient, while, in general, diesel engines are more efficient but produce more emissions.
More recently, other types of combustion methodologies have been introduced for internal combustion engines. One of these combustion concepts is known in the art as HCCI combustion. HCCI combustion, also referred to as controlled auto-ignition combustion, comprises a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry, rather than by fluid mechanics. In an engine operating in the HCCI combustion mode, the intake charge is nearly homogeneous in composition, temperature, and residual level at intake valve closing time. Because controlled auto-ignition is a distributed kinetically-controlled combustion process, the engine operates at a very dilute fuel/air mixture (i.e., lean of a fuel/air stoichiometric point) and has a relatively low peak combustion temperature, thus forming extremely low NOx emissions. The fuel/air mixture for controlled auto-ignition is relatively homogeneous, as compared to the stratified fuel/air combustion mixtures used in diesel engines, and, therefore, the rich zones that form smoke and particulate emissions in diesel engines are substantially eliminated. Because of this very dilute fuel/air mixture, an engine operating in the controlled auto-ignition combustion mode can operate unthrottled to achieve diesel-like fuel economy.
In an HCCI engine, combustion of a cylinder charge is flameless, and occurs spontaneously throughout the entire combustion chamber volume. The homogeneously mixed cylinder charge is auto-ignited as the cylinder charge is compressed and its temperature increases.
The combustion process in an HCCI engine depends strongly on factors such as cylinder charge composition, temperature, and pressure at the intake valve closing. These factors are impacted by current and recent engine operating states establishing residual energy present within the combustion chamber at the time of intended combustion. Engine operating state is frequently estimated by engine speed and engine load. Because HCCI combustion is particularly sensitive to in-cylinder conditions, the control inputs to the engine, for example, fuel mass and injection timing and intake/exhaust valve profile, must be carefully coordinated to ensure robust auto-ignition combustion.
Generally, for best fuel economy, an HCCI engine operates unthrottled and with a lean air-fuel mixture. Further, in an HCCI engine using exhaust recompression valve strategy, the cylinder charge temperature is controlled by trapping different amount of the hot residual gas from the previous cycle by varying the exhaust valve close timing. The opening timing of the intake valve is delayed preferably symmetrical to the exhaust valve closing timing relative to TDC intake. Both the cylinder charge composition and temperature are strongly affected by the exhaust valve closing timing. In particular, more hot residual gas from a previous cycle can be retained with earlier closing of the exhaust valve which leaves less room for incoming fresh air mass. The net effects are higher cylinder charge temperature and lower cylinder oxygen concentration. In the exhaust recompression strategy, the exhaust valve closing timing and the intake valve opening timing are measured by the Negative Valve Overlap (NVO) defined as the duration in crank angle between exhaust valve closing and intake valve opening.
In addition to a valve control strategy, there must be a suitable fuel injection strategy for stable combustion. For example, at a low fueling rate (for example, fueling rate<7 mg/cycle at 1000 rpm in an exemplary 0.55 liter combustion chamber), the cylinder charge may not be hot enough for a stable auto-ignited combustion in spite of the highest value of NVO allowed, leading to a partial-burn or misfire. One way to increase the charge temperature is to pre-inject a small amount of fuel when the piston approaches TDC intake during the NVO recompression. A portion of the pre-injected fuel reforms due to high pressure and temperature during the recompression, and releases heat energy, increasing the cylinder charge temperature enough for successful auto-ignited combustion of the combustion charge resulting from the subsequent main fuel injection. The amount of such auto-thermal fuel reforming is based upon the pre-injection mass and timing, generally with fuel reforming increasing with earlier pre-injection timing and greater pre-injection fuel mass.
Excessive fuel reforming decreases the overall fuel economy, and lack of fuel reforming may result in combustion instability. Thus, effective control of the reforming process benefits from accurate estimations of reforming. A method is known that estimates the amount of fuel reforming using the unique characteristic of Universal Exhaust Gas Oxygen (UEGO) sensor. A control strategy is also known to indirectly control the amount of fuel reforming in an HCCI engine by monitoring engine operating conditions including intake mass air flow and exhaust air/fuel ratio, controlling negative valve overlap to control intake airflow to achieve a desired actual air-fuel ratio for a given fueling rate, and adjusting timing of pre-injection of fuel to control the measured air-fuel ratio to a desired second air/fuel ratio smaller than the desired actual air-fuel ratio. Another method for controlling an amount of fuel reforming includes measuring in-cylinder pressures during a current combustion cycle, estimating fuel mass reformed in the current cycle based on the in cylinder pressures, utilizing the estimate of fuel mass reformed in the current cycle to project reforming required in a next cycle, and effecting control over the next cycle based on the projected reforming required in the next cycle.
At medium engine speed and load, a combination of valve profile and timing (e.g., exhaust recompression and exhaust re-breathing) and fueling strategy has been found to be effective in providing adequate heating to the cylinder charge so that auto-ignition during the compression stroke leads to stable combustion with low noise. One of the main issues in effectively operating an engine in the auto-ignition combustion mode has been to control the combustion process properly so that robust and stable combustion resulting in low emissions, optimal heat release rate, and low noise is achieved over a range of operating conditions.
A spark-ignition, direct-injection engine capable of operating in controlled auto-ignition combustion mode transitions between operating in an auto-ignited combustion mode at part-load and lower engine speed conditions and in a conventional spark-ignited combustion mode at high load and high speed conditions. There is a need to have a smooth transition between the two combustion modes during ongoing engine operation, in order to maintain a continuous engine output torque and prevent any engine misfires or partial-burns during the transitions These two combustion modes require different engine operation to maintain robust combustion. One aspect of engine operation includes control of the throttle valve. When the engine is operated in the auto-ignited combustion mode, the engine control comprises lean air/fuel ratio operation with the throttle wide open to minimize engine pumping losses. In contrast, when the engine is operated in the spark-ignition combustion mode, the engine control comprises stoichiometric air/fuel ratio operation, with the throttle valve controlled over a range of positions from 0% to 100% of the wide-open position to control intake airflow to achieve stoichiometry.
In engine operation, the engine air flow is controlled by selectively adjusting position of the throttle valve and adjusting opening and closing of intake valves and exhaust valves. Adjusting the opening, and subsequent closing, of intake and exhaust valves primarily takes the form of: phasing of opening (and subsequent closing) of the valves in relation to piston and crankshaft position; and, magnitude of the lift of the valves' opening. On engine systems so equipped, opening and closing of the intake valves and exhaust valves is accomplished using a variable valve actuation (VVA) system that may include cam phasing and a selectable multi-step valve lift, e.g., multiple-step cam lobes which provide two or more valve lift profiles. In contrast to the continuously variable throttle position, the change in valve profile of the multi-step valve lift mechanism is a discrete change, and not continuous. When a transition between steps in the selectable multi-step valve lift is not effectively controlled, unwanted disturbances in engine air flow can occur, resulting in poor combustion, including misfire or partial-burns.
HCCI combustion encompasses a lean, distributed, flameless, auto-ignition combustion process resulting in potential benefits when an engine is operating in a range of HCCI capable engine speeds and loads, as described above. However, operation of HCCI combustion is not accomplished under a fixed engine control strategy, but rather ranges of control strategies can accomplish HCCI combustion with different operational results. Also, in addition to the above mentioned valve control and fuel injection strategies, other techniques are known to benefit engine operation and extend the operability range to lower loads and temperatures, including combustion chamber designs, and different valve control and ignition strategies. Although these different technologies extend the operational limits of an HCCI engine, all have a lower operability limit where the combustion cycle is too cold to achieve auto-ignition. Additionally, each control strategy has preferred ranges of operation, and each has positive and negative aspects in comparison to other valve control and fuel injection strategies. A particular control strategy operating satisfactorily in a particular engine operating range can produce excess NOx emissions or result in unstable combustion in another particular engine operating range. An engine, operating in a range of engine speeds and loads and optimizing factors such as fuel consumption, reduction of emissions, and combustion stability can switch between control strategies depending upon engine operating conditions and balancing priorities.
HCCI modes and associate auto-ignition of the air fuel change in the combustion chamber is dependent upon conditions within the chamber. Engine stability in HCCI modes in an engine operating at low loads can be difficult to maintain. A method to improve engine stability at low loads would be beneficial.